Over 50% of human breast cancer cases show amplification and/or overexpression of c-myc oncogene and some c-Myc regulated genes, like cyclin D1. However, while some studies show an association of c-myc expression or amplification with tumor aggressiveness of this disease, other studies report the opposite. This controversy may be related to the fact that c-Myc can commit a cell to either proliferation or apoptosis: while c-Myc-promoted cell proliferation makes the tumor more aggressive, c-Myc-initiated apoptosis should result in a favorable outcome. The decision on whether c-Myc should direct a cell to proliferate or to die is made under a strong influence of transforming growth factor alpha (TGFalpha). Our previous studies have shown that mammary tumors developed from MMTV-c-myc transgenic mice further progress to some focal lesions with more aggressive features. The appearance of these 'tumor-within-a-tumor" foci makes this transgenic mouse the first animal model of mammary carcinogenesis that shows identifiable multiple-steps of tumor progression. Studies with MT-tgfalphaMMTV-c-myc dual transgenic mice show TGFalpha induces cyclin D1 as an intermediator to prevent c-Myc-initiated apoptosis, leading to an early development of a more aggressive tumor phenotype, compared to the c-myc transgenic tumor. In this project, we aim to use these transgenic animals to study whether c-Myc induces expression of e2f1 and cyclin A2 to mediate the cell proliferation and mammary tumor onset while it suppresses cyclin D1 to hamper further progression of the tumor and to initiate apoptosis. We will also explore whether TGFalpha promotes mammary carcinogenesis by inducing cyclin D1 as an intermediator to prevent c-Myc-initiated apoptosis. It will further be studied whether inhibition of both c-myc and cyclin D1 will result in a better therapeutic effect than inhibition of each gene alone on breast cancer. The long-term objective of the project is to use the information resulted from these proposed studies to direct the treatment of breast cancer patients with agents that inhibit expression of both c-myc and cyclin D1 for a better therapeutic outcome; candidate agents include modified anti-gene oligonucleotides (MAGON) and a new anticancer drug FR901228.